Lithium secondary batteries which are one example of power storage devices have been used for a variety of uses, for example, laptop personal computers, mobile phones, smartphones, and next-generation clean energy vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and electronic vehicles (EVs). Properties necessary for such lithium secondary batteries are high energy density, excellent cycle characteristics, safety in a variety of operation environments, and the like.
In many of lithium secondary batteries used widely, a nonaqueous electrolyte (also referred to as a nonaqueous electrolytic solution or simply an electrolytic solution) is used; the nonaqueous electrolyte contains an organic solvent which is liquid at room temperature, such as diethyl carbonate (DEC), ethylene carbonate (EC), dimethyl carbonate (DMC), or propylene carbonate (PC), which has a high dielectric constant and high ionic conductivity, and a lithium salt containing lithium ions.
However, the above organic solvents each have volatility and a low flash point; thus, when any of the organic solvents is used in a lithium secondary battery, the lithium secondary battery could internally short out or the internal temperature of the lithium secondary battery could increase owing to overcharging or the like, so that the lithium secondary battery would explode or catch fire.
Here, Table 1 shows comparison results of thermal stability of organic solvents and an ionic liquid used in electrolytic solutions.
TABLE 1Freezing point(melting point)(° C.)Flash point (° C.)Vapor pressure (hPa)EC2481500.12PC2421350.03DMC901811DEC1263353Ionic liquid>300—≈0
Note that the organic solvent shown in Table 1 having a lower flash point and a higher vapor pressure is more flammable. Thus, a battery containing the organic solvent has a possibility of catching fire when the internal pressure of the battery increases owing to heat generation or the battery shorts out. In contrast, ionic liquids are known to have a low possibility of catching fire and causing an explosion.
In view of the above, the use of an ionic liquid (also referred to as a room temperature molten salt) which has non-flammability and non-volatility as a nonaqueous solvent of a nonaqueous electrolyte of a lithium secondary battery has been proposed. Examples of such an ionic liquid are an ionic liquid containing ethylmethylimidazolium (EMI) cation, an ionic liquid containing an N-methyl-N-propylpyrrolidinium (P13) cation, and an ionic liquid containing an N-methyl-N-propylpiperidinium (PP13) cation (see Patent Document 1).
An example of an ionic liquid containing a cyclic quaternary ammonium cation such as a PP13 cation is an ionic liquid containing a quaternary ammonium cation having a spiro ring and an amide anion with an asymmetrical structure (e.g., fluorosulfonyl(trifluoromethylsulfonylamide) (FTA; [(FSO2)(CF3SO2)N−])) (see Patent Document 2).
Properties necessary for an ionic liquid in a lithium secondary battery are high conductivity, a low possibility of reduction in conductivity at a low temperature, a low freezing point, a low viscosity, and the like besides non-flammability and non-volatility. Note that a low temperature in this specification refers to a temperature lower than approximately 25° C. (room temperature).
The summary of the properties needed for an ionic liquid (electrolytic solution) is shown below.
TABLE 2Properties needed forelectrolytic solutionOrganic solventIonic liquidBoiling point and heat resistanceacceptable to highvery highMelting pointlowlow to very lowViscosityvery lowacceptable to lowElectrochemical stabilityhighhighFlammabilityacceptablevery lowVolatilityacceptablevery low
Patent Document 2 discloses that an ionic liquid a quaternary spiro ammonium salt containing an FTA anion has resistance to oxidation and reduction, a low viscosity, and a high conductivity and thus is suitable for an electrolytic solution of a lithium secondary battery.
Further, Patent Document 2 discloses that when an ionic liquid containing a highly symmetrical cation and generally having a high melting point has an amide anion with an asymmetrical structure, such as an FTA anion, as an anion in the ionic liquid, the melting point of the ionic liquid can be lowered.